Newbie here, with front derailleur woes

I've I've been riding my trusty Surly Long haul Trucker for about 10 years, and I've reached that delicate age where butterfly handlebars seem like a better idea than the OEM drop bars, so I decided to convert them. Obviously this required replacing the bar-end shifters. I opted for a set of Simano Deore SL-M590 shift levers, and ran into a compatibility issue with the existing Shimano Tiagra front derailleur; the heavy spring in that derailleur made it impossible to actuate the shift lever. So I replace the derailleur with a Shimano Deore M590.

The issue I've got now is that when selecting the inner ring, the derailleur cage rubs against the chain on the inside, even with the L screw completely backed out and slack in the cable. I've double-checked the derailleur vertical and angle alignment per the (really great) Park Tool videos, which didn't help. Upon close examination, the chain rub occurs ever with the derailleur fully retracted, metal-to-metal on the parallelogram travel.

So, I was wondering whether or not it would be stupid to pull the gear cluster and crank and add a tiny shim to step it out a bit. Something like this: http://bit.ly/2lrNyub

One millimeter, or even half a millimeter would be enough, but I don't want to cause a host of new problems that I won't be able to identify until I'm neck deep in the swamp.